1. Field of the Invention
The present invention relates generally to an asynchronous mobile communication system. In particular, the present invention relates to an apparatus and method for estimating an initial frequency offset.
2. Description of the Related Art
Mobile communication systems can typically be classified into synchronous systems and asynchronous systems. The synchronous systems have mainly been adopted in the United States, while the asynchronous systems have been mainly adopted in Europe.
With the recent rapid growth of the mobile communication industry, future mobile communication systems capable of supporting not only voice service but also data and image services are attracting public attention, and standardization work on the future mobile communication systems is being conducted. However, the United States and Europe which are adopting different mobile communication systems are each independently carrying out separate standardizations. The European future mobile communication system is one of the new standards and is known as a Universal Mobile Telecommunication System (UMTS).
Typically, in order to search for a Node B (or base station transceiver subsystem (BTS)), a user equipment (UE; or mobile station) comprising a mobile communication system requires the performance of frequency offset estimation and compensation for a carrier frequency. Frequency offset estimation and compensation greatly affect a Node B search time and call quality. The term “frequency offset” refers to a frequency variation occurring when a carrier frequency received from a Node B varies according to various factors (e.g., Node B signal distortion or a Doppler frequency) due to a channel environment, and a UE performs frequency offset estimation and compensation in order to match a transmission/reception frequency of the UE with a transmission/reception frequency of a Node B by removing the frequency offset.
For example, if a carrier frequency is 2.14 GHz, a carrier frequency Fr that a UE receives becomes 2.14 GHz+Δfr as a specific frequency offset Δfr is added to the carrier frequency 2.14 GHz generated in a transmission side. Therefore, the UE can normally restore a received signal by estimating and compensating for the frequency offset Δfr. Generally, the frequency offset Δfr is a value in which a frequency distortion Δfdrift—in—BTS in a Node B and a Doppler frequency ΔfD are reflected.
Frequency offset estimation by a UE is performed through an automatic frequency controller (hereinafter referred to as “AFC”). An operating principle of the AFC is to correct a frequency of a received signal by comparing a received signal's frequency Fr with a carrier frequency in which an estimated specific frequency offset Δfr is reflected and continuously compensating for a difference. A detailed implementation method will be described below.
The AFC has two limitations: it cannot trace all frequency offsets because of the limited loop bandwidth, and it must know a cell scrambling code and its timing. In other words, it can operate after completion of a cell search. However, in an asynchronous mobile communication system, because it is not possible to know a cell scrambling code before acquiring a cell, the AFC cannot be used. Therefore, estimating a frequency offset before performing cell search is required, and a temperature-based frequency estimation method used in a UE supporting a synchronous mobile communication system can be used as a method for estimating an initial frequency offset. With reference to FIG. 1, a description will now be made of a temperature-based initial frequency estimation method according to the prior art.
FIG. 1 is a block diagram illustrating a frequency offset estimation apparatus in an asynchronous mobile communication system according to the prior art. Referring to FIG. 1, the frequency offset estimation apparatus can comprise a first multiplier 101, a low pass filter (LPF) 103, an analog-to-digital converter (ADC) 105, a second multiplier 107, a frequency difference detector (FDD) 109, a temperature sensor 111, a memory 113, a controller 115, an accumulator 117, a pulse duration modulator (PDM) 119, a voltage controlled oscillator (VCO) 121 and a frequency multiplier 123.
The FDD 109 divides an output signal of the second multiplier 107 into I-channel symbols and Q-channel symbols, and detects a frequency difference by performing a specific operation (e.g., I(n)Q(n−1)−I(n−1)Q(n)) on current symbols and previous symbols. In general, a frequency difference detected through the above operation is output after detection results for 4 symbols are reflected (i.e., accumulated).
The controller 115 selects an output value from the FDD 109 and a value read from the memory 113, and outputs the selected value to the accumulator 117. As stated, in an initial state, because the AFC does not normally operate, the controller 115 reads as an initial frequency offset a value stored in the memory 113 instead of the output value of the FDD 109 and outputs the read value to the accumulator 117.
More specifically, the temperature-based frequency estimation method according to the prior art estimates an initial frequency offset by measuring a temperature by means of the temperature sensor 111 and by reading a frequency offset corresponding to the measured temperature from a table stored in the memory 113, in which relationships between temperatures and frequency offsets are stored. Therefore, the controller 115 receives an ambient temperature value of the voltage controlled oscillator 121 from the temperature sensor 111, reads a frequency offset corresponding to the measured temperature based on the relationships between temperatures and frequency offsets, previously stored in the memory 113, and outputs the read frequency offset to the accumulator 117. The accumulator 117 accumulates a currently received value to add to a previously stored value, and outputs the accumulated value to the PDM 119. The PDM 119 generates a pulse corresponding to an initial frequency offset generated according to the temperature, and outputs the generated pulse to the voltage controlled oscillator 121.
The voltage controlled oscillator 121 generates a specific oscillation frequency according to the pulse value output from the PDM 119. The oscillation frequency output from the voltage controlled oscillator 121 is subject to frequency multiplication in the frequency multiplier 123, generating a carrier frequency (i.e., radio frequency). An output of the frequency multiplier 123 is multiplied by a received signal in the first multiplier 101.
If an initial frequency offset is estimated in this manner by the initial frequency offset generated according to a temperature, estimation of a next frequency offset is performed by an AFC through the above-stated AFC operation. Meanwhile, in the initial frequency offset estimation process, because the AFC does not normally operate as stated above, the controller 115 selected the value read from the memory 113. However, when the AFC normally operates by the initial frequency offset estimation, the controller 115 selects the output value of the FDD 109. That is, as a loop of the AFC continuously performs a normal operation, although a received frequency is changed, it is possible to trace the changed frequency offset.
For the voltage controlled oscillator 121, an oven controlled temperature compensated crystal oscillator (OCTCXO) or a voltage controlled temperature compensated crystal oscillator (VCTCXO) is used as a reference frequency generator.
Meanwhile, in the asynchronous mobile communication system, the temperature-based frequency estimation method used in the synchronous mobile communication system can be used as a method for estimating an initial frequency offset before completion of a cell search. As stated, the temperature-based frequency estimation method estimates an initial frequency offset by measuring an ambient temperature of a place where the VCTCXO operates and by reading a frequency offset corresponding to the measured temperature from a table in which a relationship between temperatures and frequency offsets are stored.
However, because such a method estimates an initial frequency offset based on a table in which relationships between temperatures and frequency offsets are stored, an extra memory for storing the relationships is required, a unique table must be set up for each VCTCXO used in each UE, and the table must be changed after a run time of the VCTCXO. Actually, the VCTCXOs, though they are the same model made by the same company, show considerably different characteristics, and the characteristics vary undesirably over time.
In addition, although an initial frequency offset is estimated based on a value stored in the table in order to compensate for an influence of temperature, an initial frequency offset between a frequency transmitted by a Node B and a frequency generated by a UE to receive a signal transmitted by the Node B can show a considerably large value. Actually, in a 3rd Generation Partnership Project (3GPP) asynchronous system, when a Doppler frequency is taken into consideration, an initial carrier frequency offset is about 7.5 KHz and a system clock frequency offset is about 100 Hz.
When an initial frequency offset is considerably large, an AFC fails in frequency offset compensation and a long time is required until a frequency offset is compensated. That is, a very long time is required until a frequency offset is corrected because after a step#3 cell search is performed with an initially estimated frequency offset, if the cell search failed, an accurate frequency offset is detected by reading another frequency offset from a table in which relationships between temperatures and frequency offsets are stored and repeating the step#3 cell search for the read frequency offset. This causes call failure or a deterioration in call quality.